Human Anatomy & Physiology (11th Edition)
Human Anatomy & Physiology (11th Edition)
11th Edition
ISBN: 9780134580999
Author: Elaine N. Marieb, Katja N. Hoehn
Publisher: PEARSON
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### Urea Cycle Overview

The diagram illustrates the urea cycle, a biochemical pathway essential for removing excess nitrogen in the form of urea. The cycle primarily occurs in the liver and involves several key substances and enzymes.

#### Key Substances and Enzymes:
1. **1 - Ornithine**
2. **2 - Carbamoyl Phosphate**
3. **3 - L-Citrulline**
4. **4 - Argininosuccinate**
5. **5 - Fumarate**
6. **6 - L-Arginine**
7. **7 - Urea**

#### Enzymes Involved:
- **CPS-1 (Carbamoyl Phosphate Synthetase I)**
- **OTC (Ornithine Transcarbamylase)**
- **ASS (Argininosuccinate Synthetase)**
- **ASL (Argininosuccinate Lyase)**
- **ARG1 (Arginase 1)**

### Step-by-Step Pathway:
1. **CPS-1** catalyzes the formation of carbamoyl phosphate from 2 ATP, bicarbonate (HCO₃⁻), and NH₃.
2. **OTC** converts carbamoyl phosphate and ornithine into citrulline, which then exits the mitochondrion.
3. **ASS** catalyzes the reaction between citrulline and aspartate to produce argininosuccinate, using ATP in the process.
4. **ASL** converts argininosuccinate to arginine and fumarate.
5. **ARG1** hydrolyzes arginine to produce urea and ornithine. The urea is excreted, and ornithine is recycled back into the mitochondrion to participate in another cycle.

### Structural Representation of Urea:
The urea molecule depicted at the end is:
\[ \text{O}
     | 
\text{H}_{2}\text{N}-\text{C}-\text{NH}_{2}
 \]

### Quiz Question:

**Which of the following is a carbon input to this pathway? (Numbers refer to the above key)**

- 1
- 2
- 3
- 4
- **None of these is a carbon input**

In this case, the correct answer is **2** (Carbamoyl
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Transcribed Image Text:### Urea Cycle Overview The diagram illustrates the urea cycle, a biochemical pathway essential for removing excess nitrogen in the form of urea. The cycle primarily occurs in the liver and involves several key substances and enzymes. #### Key Substances and Enzymes: 1. **1 - Ornithine** 2. **2 - Carbamoyl Phosphate** 3. **3 - L-Citrulline** 4. **4 - Argininosuccinate** 5. **5 - Fumarate** 6. **6 - L-Arginine** 7. **7 - Urea** #### Enzymes Involved: - **CPS-1 (Carbamoyl Phosphate Synthetase I)** - **OTC (Ornithine Transcarbamylase)** - **ASS (Argininosuccinate Synthetase)** - **ASL (Argininosuccinate Lyase)** - **ARG1 (Arginase 1)** ### Step-by-Step Pathway: 1. **CPS-1** catalyzes the formation of carbamoyl phosphate from 2 ATP, bicarbonate (HCO₃⁻), and NH₃. 2. **OTC** converts carbamoyl phosphate and ornithine into citrulline, which then exits the mitochondrion. 3. **ASS** catalyzes the reaction between citrulline and aspartate to produce argininosuccinate, using ATP in the process. 4. **ASL** converts argininosuccinate to arginine and fumarate. 5. **ARG1** hydrolyzes arginine to produce urea and ornithine. The urea is excreted, and ornithine is recycled back into the mitochondrion to participate in another cycle. ### Structural Representation of Urea: The urea molecule depicted at the end is: \[ \text{O} | \text{H}_{2}\text{N}-\text{C}-\text{NH}_{2} \] ### Quiz Question: **Which of the following is a carbon input to this pathway? (Numbers refer to the above key)** - 1 - 2 - 3 - 4 - **None of these is a carbon input** In this case, the correct answer is **2** (Carbamoyl
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